The present invention relates to an agent for stimulating the growth of animal cells and a serum-free medium containing said agent. The agent is useful for stimulating the growth of animal cells, such as lymphocytes or hybridomas, which are difficult or unlikely to grow in a serum-free medium. The serum-free medium according to the present invention is employed as a medium for growing cells which produce physiologically active substances or for culturing cells for transplantation into a mammalian body.
Serum is the liquid portion that remains when blood clots spontaneously and the formed and clotting elements are removed, e.g., by centrifugation. Culture media for culturing animal cells typically contain about 5% to 20% serum in addition to amino acids, vitamins, sugars and inorganic salts. Animal cells stop growing and die in the absence of serum.
Serum is generally the most costly ingredient in culture media, accounting for about 75% to 95% of the cost. There are several drawbacks to the use of sera in culture media. For example, the quality of the sera can vary greatly between lots; sera are likely to be contaminated with Mycoplasma or viruses, and they cannot be sterilized with high-pressure steam due to the presence of serum proteins which would be denatured; and it is difficult to isolate and purify physiologically active substances produced by the animal cells cultured in serum-containing culture media due to the presence of the serum proteins. In order to overcome the disadvantages and drawbacks associated with serum-containing culture media, serum-free culture media have been employed.
A number of serum-free culture media are known, including, for example, a medium in which Eagle's MEM (Minimal Essential Medium) is used as a base medium to which albumin, insulin and/or transferrin are added; or a medium in which modified Eagle's MEM is used as a base medium, to which a factor for stimulating cells, which is obtainable by fractioning bovine serum with ammonium sulfate, is added. These serum-free media can be used in lieu of serum-containing media for culturing animal cells, however, they are less effective for growing these cells than serum-containing media. These media also are costly, and still contain residual serum elements even if fractioned. Serum-free media which are cost effective and which can be used to efficiently culture mammalian cells are needed.